This chapter introduces the C# operators related to assignment and arithmetic, shows how to use the operators along with the const keyword to declare constants, introduces the if statement and the important concepts of code blocks and scope, and discusses the bitwise operators, especially regarding masks. It also covers other control flow statements such as loops, switch, and goto, and ends with the C# preprocessor directives.

This chapter is from the book

This chapter is from the book

In this chapter, you will learn about operators, control flow statements, and the C# preprocessor. Operators provide syntax for performing different calculations or actions appropriate for the operands within the calculation. Control flow statements provide the means for conditional logic within a program or looping over a section of code multiple times. After introducing the if control flow statement, the chapter looks at the concept of Boolean expressions, which are embedded within many control flow statements. Included is mention of how integers will not cast (even explicitly) to bool and the advantages of this restriction. The chapter ends with a discussion of the C# preprocessor directives.

Operators

Now that you have been introduced to the predefined data types (refer to Chapter 2), you can begin to learn more about how to use these data types in combination with operators in order to perform calculations. For example, you can make calculations on variables that you have declared.

Beginner Topic: Operators

Operators are used to perform mathematical or logical operations on values (or variables) called operands to produce a new value, called the result. For example, in Listing 3.1 the subtraction operator, -, is used to subtract two operands, the numbers 4 and 2. The result of the subtraction is stored in the variable difference.

Listing 3.1. A Simple Operator Example

int difference = 4 – 2;

Operators are generally broken down into three categories: unary, binary, and ternary, corresponding to the number of operands 1, 2, and 3, respectively. This section covers some of the most basic unary and binary operators. Introduction to the ternary operator appears later in the chapter.

Plus and Minus Unary Operators (+, -)

Sometimes you may want to change the sign of a numerical value. In these cases, the unary minus operator (-) comes in handy. For example, Listing 3.2 changes the total current U.S. debt to a negative value to indicate that it is an amount owed.

Using the minus operator is equivalent to subtracting the operand from zero.

The unary plus operator (+) rarely2 has any effect on a value. It is a superfluous addition to the C# language and was included for the sake of symmetry.

Arithmetic Binary Operators (+, -, *, /, %)

Binary operators require two operands. C# uses infix notation for binary operators: The operator appears between the left and right operands. The result of every binary operator other than assignment must be used somehow: for example, by using it as an operand in another expression such as an assignment.

Language Contrast: C++—Operator-Only Statements

In contrast to the rule mentioned above, C++ will allow a single binary expression to form the entirety of a statement, such as 4+5;, to compile. In C#, only call, increment, decrement, and object creation expressions are allowed to be the entirety of a statement.

The subtraction example in Listing 3.3 is an example of a binary operator—more specifically, an arithmetic binary operator. The operands appear on each side of the arithmetic operator and then the calculated value is assigned. The other arithmetic binary operators are addition (+), division (/), multiplication (*), and remainder (%)—sometimes called the mod operator.

In the highlighted assignment statements above, the division and remainder operations are executed before the assignments. The order in which operators are executed is determined by their precedence and associativity. The precedence for the operators used so far is as follows.

*, /, and % have highest precedence.

+ and - have lower precedence.

= has the lowest precedence of these six operators.

Therefore, you can assume that the statement behaves as expected, with the division and remainder operators executing before the assignment.

If you forget to assign the result of one of these binary operators, you will receive the compile error shown in Output 3.2.

Output 3.2.

... error CS0201: Only assignment, call, increment, decrement,
and new object expressions can be used as a statement

When an expression contains multiple operators it can be unclear what precisely the operands of each operator are. For example, in the expression x+y*z clearly the expression x is an operand of the addition and z is an operand of the multiplication. But is y an operand of the addition or the multiplication?

Parentheses allow you to unambiguously associate an operand with its operator. If you wish y to be a summand, you can write the expression as (x+y)*z; if you want it to be a multiplicand, you can write x+(y*z).

However, C# does not require you to parenthesize every expression containing more than one operator; instead, the compiler can use associativity and precedence to figure out from the context what parentheses you have omitted. Associativity determines how similar operators are parenthesized; precedence determines how dissimilar operators are parenthesized.

A binary operator may be “left-associative” or “right-associative,” depending on whether the expression “in the middle” belongs to the operator on the left or the right. For example, a-b-c is assumed to mean (a-b)-c, and not a-(b-c); subtraction is therefore said to be “left-associative.” Most operators in C# are left-associative; the assignment operators are right-associative.

When the operators are dissimilar, the precedence for those operators is used to determine which side the operand in the middle belongs to. For example, multiplication has higher precedence than addition, and therefore, the expression x+y*z is evaluated as x+(y*z) rather than (x+y)*z.

It is often still a good practice to use parentheses to make the code more readable even when use of parentheses does not change the meaning of the expression. For example, when performing a Celsius-to-Fahrenheit temperature conversion, (c*9.0/5.0)+32.0 is easier to read than c*9.0/5.0+32.0, even though the parentheses are completely unnecessary.

Guidelines

DO use parentheses to make code more readable, particularly if the operator precedence is not clear to the casual reader.

Clearly, operators of higher precedence must execute before adjoining operators of lower precedence: in x+y*z the multiplication must be executed before the addition because the result of the multiplication is the left-hand operand of the addition. However, it is important to realize that precedence and associativity affect only the order in which the operators themselves are executed; they do not in any way affect the order in which the operands are evaluated.

Operands are always evaluated left-to-right in C#. In an expression with three method calls such as A()+B()*C(), first A() is evaluated, then B(), then C(), then the multiplication operator determines the product, and then the addition operator determines the sum. Just because C() is involved in a multiplication and A() is involved in a lower-precedence addition does not imply that method invocation C() happens before method invocation A().

Language Contrast: C++: Evaluation Order of Operands

In contrast to the rule mentioned above, the C++ specification allows an implementation broad latitude to decide the evaluation order of operands. When given an expression such as A()+B()*C(), a C++ compiler can choose to evaluate the function calls in any order, just so long as the product is one of the summands. For example, a legal compiler could evaluate B(), then A(), then C(), then the product, and then the sum.

Using the Addition Operator with Strings

Operators can also work with non-numeric operands. For example, it is possible to use the addition operator to concatenate two or more strings, as shown in Listing 3.4.

The original Tacoma Bridge in Washington
was brought down by a 42 mile/hour wind.

Because sentence structure varies among languages in different cultures, developers should be careful not to use the addition operator with strings that require localization. Composite formatting is preferred (refer to Chapter 1).

Guidelines

DO favor composite formatting over the addition operator for concatenating strings.

Using Characters in Arithmetic Operations

When introducing the char type in the preceding chapter, we mentioned that even though it stores characters and not numbers, the char type is an integral type (“integral” means it is based on an integer). It can participate in arithmetic operations with other integer types. However, interpretation of the value of the char type is not based on the character stored within it, but rather on its underlying value. The digit 3, for example, contains a Unicode value of 0x33 (hexadecimal), which in base 10 is 51. The digit 4, on the other hand, contains a Unicode value of 0x34, or 52 in base 10. Adding 3 and 4 in Listing 3.5 results in a hexadecimal value of 0x167, or 103 in base 10, which is equivalent to the letter g.

Listing 3.5. Using the Plus Operator with the char Data Type

You can use this trait of character types to determine how far two characters are from each other. For example, the letter f is three characters away from the letter c. You can determine this value by subtracting the letter c from the letter f, as Listing 3.6 demonstrates.

Listing 3.6. Determining the Character Difference between Two Characters

int distance = 'f' – 'c';
System.Console.WriteLine(distance);

Output 3.5 shows the results of Listing 3.6.

Output 3.5.

3

Special Floating-Point Characteristics

The binary floating-point types, float and double, have some special characteristics, such as the way they handle precision. This section looks at some specific examples, as well as some unique floating-point type characteristics.

A float, with seven decimal digits of precision, can hold the value 1,234,567 and the value 0.1234567. However, if you add these two floats together, the result will be rounded to 1234567, because the exact result requires more precision than the seven significant digits that a float can hold. The error introduced by rounding off to seven digits can become large compared to the value computed, especially with repeated calculations. (See also the upcoming Advanced Topic, Unexpected Inequality with Floating-Point Types.)

Since the double can hold a more accurate value than the float can store, the C# compiler will actually evaluate this expression to double number = 140.600006103516 because 140.600006103516 is the closest binary fraction to 140.6 as a float. This fraction is slightly larger than 140.6 when represented as a double.

The Assert() methods alert the developer whenever their argument evaluates to false. However, of all the Assert() calls in this code listing, only half have arguments that evaluate to true. In spite of the apparent equality of the values in the code listing, they are in fact not equivalent due to the inaccuracies of a float.

Guidelines

AVOID using equality conditionals with binary floating-point types. Either subtract the two values and see if their difference is less than a tolerance, or use the decimal type.

You should be aware of some additional unique floating-point characteristics as well. For instance, you would expect that dividing an integer by zero would result in an error, and it does with data types such as int and decimal. The float and double types instead allow for certain special values. Consider Listing 3.8, and its resultant output, Output 3.7.

Listing 3.8. Dividing a Float by Zero, Displaying NaN

float n=0f;
// Displays: NaN
System.Console.WriteLine(n / 0);

Output 3.7.

NaN

In mathematics, certain mathematical operations are undefined, including dividing zero by itself. In C#, the result of dividing the float zero by zero results in a special “Not a Number” value; all attempts to print the output of such a number will result in NaN. Similarly, taking the square root of a negative number with System.Math.Sqrt(-1) will result in NaN.

A floating-point number could overflow its bounds as well. For example, the upper bound of the float type is approximately 3.4 x 1038. Should the number overflow that bound, the result would be stored as “positive infinity” and the output of printing the number would be Infinity. Similarly, the lower bound of a float type is –3.4 x 1038, and computing a value below that bound would result in “negative infinity,” which would be represented by the string -Infinity. Listing 3.9 produces negative and positive infinity, respectively, and Output 3.8 shows the results.

Listing 3.9. Overflowing the Bounds of a float

Further examination of the floating-point number reveals that it can contain a value very close to zero, without actually containing zero. If the value exceeds the lower threshold for the float or double type, the value of the number can be represented as “negative zero” or “positive zero,” depending on whether the number is negative or positive, and is represented in output as -0 or 0.

Compound Assignment Operators (+=, -=, *=, /=, %=)

Chapter 1 discussed the simple assignment operator, which places the value of the right-hand side of the operator into the variable on the left-hand side. Compound assignment operators combine common binary operator calculations with the assignment operator. Take Listing 3.10, for example.

Listing 3.10. Common Increment Calculation

int x = 123;
x = x + 2;

In this assignment, first you calculate the value of x + 2 and then you assign the calculated value back to x. Since this type of operation is relatively frequent, an assignment operator exists to handle both the calculation and the assignment with one operator. The += operator increments the variable on the left-hand side of the operator with the value on the right-hand side of the operator, as shown in Listing 3.11.

Listing 3.11. Using the += Operator

int x = 123;
x += 2;

This code, therefore, is equivalent to Listing 3.10.

Numerous other combination assignment operators exist to provide similar functionality. You can also use the assignment operator with subtraction, multiplication, division and the remainder operators (Listing 3.12 demonstrates).

Listing 3.12. Other Assignment Operator Examples

x -= 2;
x /= 2;
x *= 2;
x %= 2;

Increment and Decrement Operators (++, --)

C# includes special unary operators for incrementing and decrementing counters. The increment operator,++, increments a variable by one each time it is used. In other words, all of the code lines shown in Listing 3.13 are equivalent.

Listing 3.13. Increment Operator

spaceCount = spaceCount + 1;
spaceCount += 1;
spaceCount++;

Similarly, you can also decrement a variable by one using the decrement operator,--. Therefore, all of the code lines shown in Listing 3.14 are also equivalent.

Listing 3.14. Decrement Operator

lines = lines - 1;
lines -= 1;
lines--;

Beginner Topic: A Decrement Example in a Loop

The increment and decrement operators are especially prevalent in loops, such as the while loop described later in the chapter. For example, Listing 3.15 uses the decrement operator in order to iterate backward through each letter in the alphabet.

Listing 3.15. Displaying Each Character’s Unicode Value in Descending Order

The increment and decrement operators are used to control how many times a particular operation is performed. Notice also that in this example, the increment operator is used on a character (char) data type. You can use increment and decrement operators on various data types as long as some meaning is assigned to the concept of the “next” or “previous” value for that data type.

We saw that the assignment operator first computes the value to be assigned, and then causes the assignment. The result of the assignment operator is the value that was assigned. The increment and decrement operators are similar: They compute the value to be assigned, perform the assignment, and result in a value. It is therefore possible to use the assignment operator with the increment or decrement operator, though doing so carelessly can be extremely confusing. See Listing 3.16 and Output 3.10 for an example.

Listing 3.16. Using the Post-Increment Operator

You might be surprised that result was assigned the value that was countbeforecount was incremented. Where you place the increment or decrement operator determines whether the assigned value should be the value of the operand before or after the calculation. If you want the value of result to be the value assigned to count, you need to place the operator before the variable being incremented, as shown in Listing 3.17.

In this example, the increment operator appears before the operand, so the result of the expression is the value assigned to the variable after the increment. If count is 123, ++count will assign 124 to count and produce the result 124. By contrast, the postfix increment operator count++ assigns 124 to count and produces the value that count held before the increment: 123. Regardless of whether the operator is postfix or prefix, the variable count will be incremented before the value is produced; the only difference is which value is produced. The difference between prefix and postfix behavior appears in Listing 3.18. The resultant output is shown in Output 3.12.

As Listing 3.18 demonstrates, where the increment and decrement operators appear relative to the operand can affect the result produced by the expression. The result of the prefix operators is the value that the variable had before it was incremented or decremented. The result of the postfix operators is the value that the variable had after it was incremented or decremented. Use caution when embedding these operators in the middle of a statement. When in doubt as to what will happen, use these operators independently, placing them within their own statements. This way, the code is also more readable and there is no mistaking the intention.

Language Contrast: C++—Implementation-Defined Behavior

Earlier we discussed how in C++, the operands in an expression can be evaluated in any order, whereas in C# they are always evaluated left to right. Similarly, in C++ an implementation may legally perform the side effects of increments and decrements in any order. For example, in C++ a call of the form M(x++, x++) where x begins as 1 can legally call M(1,2) or M(2,1) at the whim of the compiler; C# will always call M(1,2) because C# makes two guarantees: first, that the arguments to a call are always computed left to right, and second, that the assignment of the incremented value to the variable always happens before the value of the expression is used. C++ makes neither guarantee.

Guidelines

AVOID confusing usages of the increment and decrement operators.

DO be cautious when porting code between C, C++, and C# that uses increment and decrement operators; C and C++ implementations need not follow the same rules as C#.

Advanced Topic: Thread-Safe Incrementing and Decrementing

In spite of the brevity of the increment and decrement operators, these operators are not atomic. A thread context switch can occur during the execution of the operator and can cause a race condition. You could use a lock statement to prevent the race condition. However, for simple increments and decrements, a less expensive alternative is to use the thread-safe Increment() and Decrement() methods from the System.Threading.Interlocked class. These methods rely on processor functions for performing fast thread-safe increments and decrements. See Chapter 19 for more details.

Constant Expressions and Constant Locals

The preceding chapter discussed literal values, or values embedded directly into the code. It is possible to combine multiple literal values in a constant expression using operators. By definition, a constant expression is one that the C# compiler can evaluate at compile time (instead of calculating it when the program runs) because it is composed entirely of constant operands. Constant expressions can then be used to initialize constant locals, which allow you to give a name to a constant value (similar to the way local variables allow you to give a name to a storage location). For example, the computation of the number of seconds in a day can be a constant expression that is then used in other expressions by name.

The const keyword in Listing 3.19 declares a constant local. Since a constant local is by definition the opposite of a variable—“constant” means “not able to vary”—any attempt to modify the value later in the code would result in a compile-time error.

Guidelines

DO NOT use a constant for any value that can possibly change over time. The value of pi and the number of protons in an atom of gold are constants; the price of gold, the name of your company, and the version number of your program can change.

Note that the expression assigned to secondsPerWeek is a constant expression because all the operands in the expression are also constants.